Radial turbine rotors or wheels in gas turbine engines are subjected to very high temperatures, severe thermal gradients, and very high centrifugal forces. The turbine blades are located directly in and are directly exposed to the hot gas-stream. The inducer tips of the blades therefore experience the highest temperatures and consequently are most susceptible to creep rupture failure that could result in an inducer tip striking the surrounding nozzle enclosure, causing destruction of the turbine. The turbine hub is subjected to very high radial tensile forces and also has a life limit imposed by low-cycle-fatigue crack initiation and growth. In order to achieve optimum blade and hub material properties, dual alloy structures have been developed in which the hub portion is formed of wrought superalloy material having high tensile strength and high low-cycle fatigue strength, while the blade ring portion, including the blades (i.e., airfoils) and blade rim, is formed of a cast superalloy material having high creep rupture strength at very high temperatures. The dual alloy approach has been used where very high performance turbine rotors are required because those materials that have optimum properties for the turbine blades do not have sufficiently high tensile strength and sufficiently high low-cycle fatigue strength for use in the turbine hubs.
U.S. Pat. No. 4,581,300 issued Apr. 8, 1986 to Hoppin et al and U.S. Pat. No. 4,659,288 issued Apr. 21, 1987 to Clark et al, both assigned to the assignee of the present invention, disclose methods for manufacturing a turbine rotor from two portions each having a different superalloy composition. The disclosures of said patents are incorporated herein by reference to aid in understanding the background of the present invention.
One problem in manufacturing such dual alloy components is in selecting the proper heat treating cycle to optimize the mechanical properties of both superalloy components. Obviously, selecting the thermal treatment employed to maximize strength of one of the alloys would not be expected to be optimum for a component containing a second alloy. Further, it would be apparent to those skilled in this art that merely "splitting the difference" between the temperatures and times of the two alloys' usual thermal treatment would be even less satisfactory and may even be totally useless (i.e., both components may have poor mechanical properties).
The aforementioned U.S. Pat. No. 4,659,288 teaches one method to heat treat a dual alloy turbine wheel in column 6, lines 5 to 35. However, the procedure is lengthy (about 36 to 48 hours) and complex (5 furnace cycles). In view of the foregoing, it should be apparent that there is an unmet need in the art for improvements in the heat treating of dual alloy components for use as turbine rotors in high performance gas turbine engines.
It is therefore an object of the present invention to provide a novel method for improving the mechanical properties of certain dual alloy components. It is a further object of the present invention to provide a new and improved method of heat treating alloy turbine rotors for use in high performance gas turbine engines.